The differentiation of mesenchymal progenitor cells (MPCs) in the bone marrow environment is a crucial determinant of bone mass, whole body metabolism, and bone repair. However, the mechanisms that regulate MPC differentiation are poorly understood. This proposal is based on the hypothesis that MPCs serve to sense metabolic stress which then directs their differentiation. In this view, the control of adipogenic vs. osteogenic differentiation of MPCs is a mechanism for balancing structural/biomechanical and metabolic needs. Excessive marrow adipogenesis is associated with bone loss and metabolic adaptation whereas excessive osteoblast differentiation favors acquisition of bone mass possibly at the cost of reduced capacity for metabolic adaptation. Conditions of high metabolic stress, such as diet- induced obesity, drive adipogenic differentiation in preference to osteoblast differentiation. We propose that G protein signaling is a key participant in metabolic sensing in MPCs, with the inhibitory G protein (Gi) signaling pathway promoting the production of reactive oxygen species (ROS) that drives increased marrow adipogenesis and bone loss. These concepts will be tested in three specifics aims. In specific aims 1 and 2, we will utilize mouse genetic models to effect constitutive Gi signaling (Aim 1) or blockade of Gi signaling (Aim 2) in MPCs. We will investigate the effect of these manipulations on bone marrow adipose tissue, bone mass, fracture repair, and adaption to metabolic stress (high fat diet). In specific aim 3, we will use cellular and molecular approaches to elucidate the mechanisms by which MPC differentiation is regulated by Gi signaling and by metabolic stress. Successful completion of these studies will identify novel mechanisms for the control of skeletal and metabolic homeostasis, will provide new insights into the basis for the well-established relationship between bone mass and bone marrow adipose tissue, and may provide new opportunities for developing therapies aimed at improving bone and metabolic health.